1. Technical Field
The present invention relates to a tuning circuit which adjusts a tuning frequency of a system applying a received signal to an inductor and changing a capacitance of a tuning variable capacitor unit connected to the inductor to obtain a tuning signal at the inductor.
2. Background Art
In an AM radio, a signal has traditionally been supplied from an antenna to an antenna tuning circuit, where a tuning signal centered on a frequency of the desired channel has been extracted. In one tuning circuit, a variable capacitor is manually operated to change the tuning frequency. In this case, the tuning frequency is changed while checking an output sound from a speaker to set the tuning frequency to be the desired one.
On the other hand, digital adjustment of the tuning frequency has increasingly been used, and in this case, the tuning frequency is adjusted by switching capacitor banks to adjust a capacitance depending on a signal for the desired channel frequency. At this time, the capacitor banks are switched to connect the inductor to which a received signal is input in a resonant mode such that a resonant frequency becomes the desired channel frequency. In this case, an oscillation frequency can be adjusted so as to follow the desired channel frequency using a phase-locked loop (PLL), a frequency-locked loop (FLL) and the like.
Since a signal not having a frequency of the desired channel is oscillated when a mode is switched to an oscillation mode to adjust the oscillation frequency, a noise on adjusting a local oscillation frequency (LocalOSC setting noise) or a noise of an antenna tuning signal (ANT tuning noise) is large. Therefore, the output is muted so that a noise from the speaker may not be increased.
Thus, a user cannot adjust the tuning frequency in such a tuning circuit while hearing the sound output from the speaker.